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National Advisory Committee for Aeronautics, Technical Notes - Flight Investigation of Effects of Retreating Blade Stall on Bending and Torsional Moments Encountered by a Helicopter Rotor Blade

Flight tests have been conducted with a medium-size single-rotor
helicopter, one blade of which was equipped with strain gages, to deter-
mine the effects of retreating-blade stall on the rotor blade bending
and torsional moments during high-speed flight and pull-up maneuvers.

The results indicate that retreating-blade stall has a substantial
effect on the periodic rotor blade moments. In the stalled condition,
the higher harmonics can become almost as large numerically and as impor-
tant from a fatigue standpoint as the lower harmonics. Since these
increased moments, particularly the torsional moments, cause increased
periodic control loads and increased vibration in the helicopter control
system, they may tend to restrict the normal operating limits of future
high-performance helicOpters unless an adequate combination of means to
reduce these moments and means to design for their fatigue effects is
devised.

A key item in obtaining increased reliability, reduced maintenance
time, and reduced overall cost of helic0pters, together with light struc-
tural weight, is the designing of the various components to avoid fatigue
failures. Periodic moments in the rotor blade and the resultant loads in
the control systems can lead to failure of these very critical parts unless
they are designed to withstand their cumulative effects. Currently, the
helicopter industry has encountered increased periodic control loads in
high-performance prototype helicopters during high-Speed flight and esPe—
cially during high-speed maneuvers. These increased loads have seriously
restricted.the normal operating limits of these prototypes and are a mat-
ter of great concern.

The increased periodic control loads are a result of increased tor—
sional moments about a spanwise axis. These_increased torsional moments
are not as critical as the bending moments in determining the fatigue life
of the blade itself, but they are of prime importance in the control-system
design. Since the results presented in reference 1 have shown that atmos-
pheric turbulence and moderate control motions contribute little to the
total torsional and bending moments, retreating-blade stall was suspected
of being a primary cause of these increased moments in high-speed level
flight and high-speed maneuvers.